>>6417658Repliroids do not comply with the Laws of Robotics by design. That's why Light was so hesitant to release X upon the world in the first place. X is a sapient robot with a complete free will, no strings attached.
Irregulars are called Irregulars because they behave outside what human society deems fitting behaviour for Repliroids and nothing else.
In the X series, going postal on civilians or causing large scale property damage would both fit the bill for being called Irregular. In the Zero series this is taken to its extreme point, where under the rule of Copy X any Repliroid that fails to bow down to X's absolute will is deemed Irregular.
The Zero and Sigma virus aren't the cause of Irregularity, they are simply a catalyst or accelerant. They inhibit higher level thought functions, while allowing lower level functions to operate uninhibited. This leads to an increase in violent tendencies and a decrease in logical and moral reasoning. For a run-of-the-mill Repliroid, the result is simple violent outbursts. In the case of Sigma's advanced brain circuitry, you get a genocidal maniac instead.